1. Technical Field
The present invention relates to an optical filter device.
2. Related Art
In related art, there has been a transmission wavelength tunable interference filter (hereinafter, referred to as “tunable etalon”) disclosed in Patent Document 1 (JP-A-11-142752). The tunable etalon varies the wavelength to transmit by adjusting a gap between mirrors using an external force on opposed substrates, for example, an electrostatic actuator. In the transmittance characteristics of the tunable etalon for wavelengths, in wavelength bands divided at fixed intervals, transmittance is determined at each interval. The wavelength interval is determined depending on the value of an electrostatic force (voltage) applied to the tunable etalon. Therefore, to obtain spectrum data of colors of objects, it is necessary to obtain data while changing the voltage applied to the tunable etalon sequentially with respect to each wavelength interval.
FIG. 9 shows a block diagram of an optical filter device that measures a light reception voltage in a wavelength band using a tunable etalon in related art. In FIG. 9, a control arithmetic circuit 112 includes a microprocessor 111 and a voltage setting memory 110 that stores a V-λ table (voltage data table) 109. The level of the drive voltage of the tunable etalon is determined based on the voltage data of the V-λ table 109. The voltage setting memory 110 may be built in the microprocessor 111. The control arithmetic circuit 112 connects to a tunable etalon driver circuit 113, an amplifier circuit 107, and an AD converter 108.
A flow of measurement for one wavelength in an optical filter device in related art is as follows.
Measurement Procedures in Related Art
The tunable etalon driver circuit 113 outputs drive voltage data with respect to each wavelength to an electrostatic actuator of a tunable etalon 4 in response to the V-λ table 109 stored in the voltage setting memory 110 by a command from the control arithmetic circuit 112 (step 51).
A reflected light 2 or a transmitted light 3 from an object to be measured is transmitted through the tunable etalon 4 and enters a light receiving device 5 (step 52).
The light receiving device 5 is a current output device such as a photodiode, and the light is converted into a voltage (light reception voltage) in an I-V converter circuit 106 connected to the device (step 53).
The light reception voltage is amplified by the amplifier circuit 107 connected to the output of the I-V converter circuit (current-voltage converter circuit) 106 (step 54).
The amplified light reception voltage is converted from an analog signal into a digital signal by the AD converter 108 connected to the output of the amplifier circuit 107 (step 55).
The light reception voltage converted into the digital signal is measured by the microprocessor 111 (step 56).
In the case where the light reception voltage measured by the microprocessor 111 is larger than a reference voltage value, gain of the amplifier circuit 107 is lowered by a command from the control arithmetic circuit 112 so that the light reception voltage may be equal to or less than the reference voltage value, the measurement procedures step 52 to step 56 are repeatedly performed, and the measured light reception voltage value is updated (step 57).
However, there are two problems in light reception voltage measurement in the optical filter device using the tunable etalon in related art. The first problem is that the measurement time is longer. This is because the measurement procedures step 52 to step 56 are repeatedly performed until the voltage becomes equal to or less than the reference voltage value in the above described light reception voltage measurement. The second problem is that measurement accuracy becomes lower. This is because a system of raising the gain of the amplifier circuit when the measured light reception voltage is smaller is not incorporated, and, in the case where the voltage is significantly smaller than the dynamic range of the AD converter, the conversion error becomes larger at conversion from the analog signal into the digital signal.